The present invention relates to organic polyisocyanate binder compositions for the preparation of lignocellulosic molded articles and more particularly to a method for preparation of particle board with a polyisocyanate binder system having an internal release agent.
The molding of lignocellulosic and lignocellulosic-containing fibers, particles or layers to form composite articles is known. Organic di- and polyisocyanates as useful binders for lignocellulose materials have been known for some time and give to particle board products increased stability and mechanical strength; see, for example, U.S. Pat. Nos. 3,428,592; 3,440,189; 3,557,263; 3,636,199; 3,870,665; 3,919,017 and 3,930,110. It is also known in the art that the isocyanate binders are mixed with the wood particles utilized as the base for the particle board. A wood chip or particle and isocyanate binder mixture is then formed into a mat and molded with pressure and temperature to the desired size or shape. However, due to the non-aqueous nature of straight isocyanate binder systems, as well as bonding efficiency and high cost, attempts have been made to develop water emulsion polyisocyanate binder systems. The water emulsion binders allow easier cleaning of equipment and application of larger binder volumes to increase the distribution efficiency of the binder without increasing cost. Such aqueous emulsion polyisocyanates and their preparation have been described for example in U.S. Pat. Nos. 3,996,154 and 4,143,014 as well as German Offen. No. 2,724,363. A principal disadvantage of the use of the isocyanates in the preparation of particle board is due to its excellent adhesive properties. Thus, water emulsion isocyanate systems, like the straight polyisocyanate binders, stick tenaciously to the metal caul plates which are employed to support the wood particles during transporting and the pressing or molding processes. Such poor release of the molded particle board from the caul or mold surface creates difficulty in the cleaning and automatic handling of the cauls. In order to prevent the sticking problem, external release agents have been applied to the caul plates or platens or mat surface as described for example in U.S. Pat. No. 4,110,397. The external type of release agent is not only inconvenient to use because it generally must be applied for every pressing cycle, but is also not very reliable in large scale production. Another method to overcome the sticking problem has been to overlay the isocyanate bound lignocellulosic particles with a veneer of wood as shown, for example, in U.S. Pat. Nos. 3,390,110; 4,197,219 and 3,919,017, or with the use of a release paper. These methods have the disadvantage of either adding more cost to the product or of not fully utilizing the superiority of the isocyanate binder. In an attempt to minimize these disadvantages, internal release agents have been developed for incorporation into the binder or to be sprayed separately onto the wood particles or chips as described, for example in U.S. Pat. Nos. 3,870,665; 4,257,995; 4,376,088 and 4,376,089 and British Pat. No. 2,031,914.
The present invention which comprises an aqueous emulsion isocyanate binder system with a water-emulsifiable internal release agent consisting of a carboxy functional siloxane for the preparation of compression molded lignocellulosic compositions avoids the above-mentioned disadvantages.